How do they make the reflecting surface of giant telescopes barabolic mirrors?

For the glass base, they make it rotate in an oven, but how do they make the perfectly reflecting surface after?
I was also surprised by doug's post. I am preatty sure that it makes a parabola cause I eared it many times but I red the demo on the link you gave Lioni and i just don't get why they say Ekin = Ecin

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